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Microbial metabolism influences microplastic perturbation of dissolved organic matter in agricultural soils
Summary
Researchers studied how microplastics from both traditional polyethylene and biodegradable polylactic acid plastics change the chemistry of dissolved organic matter in farm soil. Soil microbes broke down substances released by the plastics, altering the soil's chemical composition over 100 days. Surprisingly, the biodegradable plastic released compounds that soil bacteria could more readily use, and after aging, it had roughly 10 times the pollutant-absorbing capacity of polyethylene, suggesting that so-called biodegradable plastics may pose their own environmental risks in agricultural soil.
An estimated 258 million tons of plastic enter the soil annually. Joining persistent types of microplastic (MP), there will be an increasing demand for biodegradable plastics. There are still many unknowns about plastic pollution by either type, and one large gap is the fate and composition of dissolved organic matter (DOM) released from MPs as well as how they interact with soil microbiomes in agricultural systems. In this study, polyethylene MPs, photoaged to different degrees, and virgin polylactic acid MPs were added to agricultural soil at different levels and incubated for 100 days to address this knowledge gap. We find that, upon MP addition, labile components of low aromaticity were degraded and transformed, resulting in increased aromaticity and oxidation degree, reduced molecular diversity, and changed nitrogen and sulfur contents of soil DOM. Terephthalate, acetate, oxalate, and L-lactate in DOM released by polylactic acid MPs and 4-nitrophenol, propanoate, and nitrate in DOM released by polyethylene MPs were the major molecules available to the soil microbiomes. The bacteria involved in the metabolism of DOM released by MPs are mainly concentrated in Proteobacteria, Actinobacteriota, and Bacteroidota, and fungi are mainly in Ascomycota and Basidiomycota. Our study provides an in-depth understanding of the microbial transformation of DOM released by MPs and its effects of DOM evolution in agricultural soils.